L-78 engine Birth date

[nuch_ss396]

Pete,

My research shows that they used the same camshaft between
the two years I mentioned above.

Anders,

I expect that the exhaust has maybe 20-25hp gain. Does the
TI ignition add the remainder of the hp gain?

Steve

[3macs1]

Here is what I got one time when I asked this question.
Insurance concerns plus General Motors Corporate policies resulted in the use of a lower RPM rating resulting in decrease from 425 HP to 375 HP as a running change for all models except corvette. The L-78 was released for sale in the 1966 SS-396 Chevelle in March 1966 with a rating of 375 HP due to this lower RPM rating and somewhat more restrictive exhaust manifolds, it was otherwise identical to the 425 HP '65 Corvette.( same cam)
The 1965 Z-16 396 Chevelle had the same engine except the Z-16 used a hydraulic camshaft which effectively limited RPM to the lower 375 HP rating.(L-37)
Also all of my 66 Impala/ Biscayne l-72 427's came with the
3880827 and 828 vet exhaust manifolds originally.

[Paul D]

My '66 Impala SS L72 also came with the Corvette cast iron exhaust manifolds.

[WILMASBOYL78]

Wasn't the L72 rated at 450hp initially and then lowered to 425hp....??? Read an interesting article the other day about a dyno test on the original 65 L78 and then revised results following a blueprint by Bill Thomas...these motors seemed to need a little help to achieve the factory ratings...

Neat stuff..wilma

[3macs1]

Yes sir it was on the l-72. I have never heard of a 66 chevelle however even the early ones being rated at 425.
learned something new today

[Salvatore]

I think GM changed ratings at times with a stroke of a pen. A lot depended on insurance companies back in those days and the cars they wanted to promote.

[JRSully]

I recently had a 70 L78 rebuilt dead stock except for a .030 overbore (70 396'a started life as a 402) 11:1 compression (verifed) nos 143 cam, 569 intake, 4557 holley, stock distributor, stock, uncut heads (hardened seats)stock valve size. Motor was pretty much clearenced and balanced. 422 hp @ 5900 WITH headers, 400 lbs torque on the Dyno. Everything is essentially the same as the 65-69 L78 except the intake (flatter, slightly lower performance) and the I think the 291 heads (70) may have had slightly larger valves vs. the 840 and also what equates to a .060 over bore over an actual 396 inch motor WITH headers, so who knows for sure..

[Stefano]

I have never Dynoed the different exhaust manifolds side by side but it would be no where near a 50 hp difference.

The top dog prior to the mystery motor was the 409/425. GM could not go backwards in the public eye/marketing, at least at the initial release of their new solid lifter engine.

Some early '66 L78 Chevelles have been documented as showing the L78 option rated at 425hp.

I believe Bill Thomas' Blue printing guide/book has a dyno chart or info showing that the L78 could produce 425hp if brought to exact specs.

The First 427/L72s were factory rated at 450hp.